In the art of positioning elements with servo means workers realize that velocity control is too apt to be deficient, e.g. especially where the servo controls the access and transport of sheet items like documents, and such items can present "friction loading" that varies widely, and can contribute to (steady-state) error in velocity determinations. An object hereof is to ameliorate such and virtually eliminate such friction-caused velocity error by measuring the friction load and using this information to adaptively change an "offset" to compensate for the error.
Often, it is desired to drive a device at very constant velocity to maintain throughput or functional accuracy. For this, a "proportional feedback control" system can be employed. However, such a system produces a steady state velocity error because of friction or other non-inertial loads. An example is a DC motor driven document feeder which separates documents from a stack and feeds them one at a time into a transport for further processing. The load put on the system by the documents is often large and variable, due to differing friction produced by different documents.
Similarly, a sorter used to process both personal checks and flimsy remittance documents would have to deal with largely differing friction loads because of friction differences between these documents. The larger friction would slow the feeder speed, decreasing throughput.
Also, the user of such machines may increase friction load for specific types of documents that may be difficult to separate, again decreasing throughput.
What is New or Different:
This invention adds hardware and/or software to a semi-conventional, "proportional velocity feedback control" system that calculates the required adjustments to control system parameters to eliminate steady state velocity error when loads are changed.
More conventional practice would call for very high gain control (systems). But high gain motor control systems may excite mechanical resonances which can render a system too acoustically noisy, cause mechanical fatigue damage, or prevent normal functioning. Another approach is to employ an integrator in the forward loop. But such integrators slow system response; i.e., acceleration and deceleration. Also, one may destabilize a control system by designing it to produce quick response, and so "integrators" typically require a control system with design compromises that may be undesirable.
This invention addresses such shortcomings and virtually eliminates velocity error without high gain and without employing an integrator, thus avoiding their undesirable effects.